1. Field of Invention
This invention pertains to monitoring voltage or electric field, and more particularly to precisely monitoring changes in the voltage by observing differences in light transmission through crystalline material which is stressed as a result of being affixed to material undergoing electrostrictive or piezoelectric structural change in response to electrical stimulation. Alternatively, the voltage changes may be monitored by observing differences in light transmission through certain crystalline material which, itself, exhibits piezoelectric or electrostrictive behavior.
2. Description of the Related Art
Voltage measurement is well known. Many typical forms of voltage analysis, including those employing common analog and digital methods, however, are not suited to certain technical applications because of problems associated with the wiring and electrical connectors that characterize those methods. Ordinary electrical wiring and components, for example, may generate (or be susceptible to) electromagnetic interference. Also, because of the possibility of creating sparks, standard electrical circuitry can pose hazards in potentially explosive environments.
Fiber-optic technology has been developed partly in response to the need for both energetic and sensing components that do not exhibit these shortcomings. Because of their unique structure, optical fibers are capable of highly accurate transmission of light, which is relatively unaffected by interference or other signal degrading phenomena. They are also generally safe in confined places where an explosion hazard exists.
Optical voltage meters are known and used. These employ various principles including the electro-optic (Pockels) effect and the alteration of light transmission through optical fibers by coating the fibers with an electrostrictive substance and then subjecting them to the effects of an electric field. Although these types of optical voltage meters are greatly superior to meters relying on ordinary electrical circuitry in many applications, they are not without certain limitations. Meters using optical fibers with electrostrictive coating, for example, often require that the fibers be very long in order to optimize the cumulative nature of the electrostrictive effect. The electro-optic effect has been demonstrated in the context of interferometry but may not be useful for some other purposes.
The present invention differs from the related art in that it utilizes the capacity of certain materials, used as substrates, to undergo a structural change in response to electrical stimuli. It also utilizes the strain dependence of optical absorption at the band edge of a semiconductor material which, in one type of application, is attached to the substrate. In the case of a piezoelectric ceramic substrate, used in one embodiment, electrical stimulation causes strain in it that is proportional to the applied field. This effects a characteristic change in the degree to which light passes through the attached semiconductor material at the band edge. For an electrostrictive ceramic substrate, used in a different embodiment of this invention, electrical stimulation creates a strain in the substrate that is quadratic, and a different characteristic change in light transmissibility of the attached semiconductor material occurs.
Changes in optical properties of crystalline materials have previously been used to detect and quantify physical strain. For example, Brogardh (U.S. Pat. No. 4,270,050, "Apparatus for Measuring Pressure Change By Absorption Spectrum Change") claimed an optical device for measuring physical forces by analyzing changes in optical transmission through a crystalline modulator subjected to those forces. Likewise, the inventor of the present invention independently described using the unique optical characteristics of gallium arsenide semiconductor material to measure physical forces ("Gallium Arsenide as an Optical Strain Gauge," J. D. Weiss, S. S. Lopez, and A. J. Howard, SAND 94-0710J, Sandia National Laboratories, Mar. 15, 1994, submitted for publication to Applied Optics).
The principle underlying these applications involves the fact that a shift occurs in the absorption edge of the light absorption spectrum of certain crystalline materials when they are subjected to compressive or tensile strain. By measuring the degree to which light is capable of passing through a suitable material and comparing it to standards for that material measured under a variety of known physical conditions, one may monitor with great sensitivity the magnitude of compressive or tensile strain to which the material is subjected under test conditions.
FIG. 1 is a graph illustrating the rapid change in optical absorption at the band edge for gallium arsenide semiconductor material. Gallium arsenide is but one of many materials which exhibit this behavior. The graph shows a comparison of band-edge wavelengths and absorption coefficients for gallium arsenide under unstrained, compressed and extended conditions. In an unstrained state, the band edge occurs in the vicinity of 900 nm. For light of a fixed optical wavelength passing through the material, an increase in transmission under compression, and a decrease in transmission under tension is observed. This strain-induced change in optical transmission can be substantial when the band edge is steep. Thus, gallium arsenide can be used as an optical strain gauge.
FIG. 2 illustrates an experimental arrangement whereby gallium arsenide may be used as a strain gauge. The figure shows a chip of gallium arsenide semiconductor material 5 affixed to the surface of a substrate 10 which is to be subjected to strain. The substrate could be a cantilevered beam, for example, whose relevant strain is in the direction shown by the arrow 15. An input optical fiber 20 carries light from an optical source 25 to the surface of the semiconductor material 5 where it enters the semiconductor material and passes through. The amount of light which passes through the semiconductor material depends on the optical absorption at the band edge for that material. Light then exits the gallium arsenide crystal and is collected by an output optical fiber 30 which carries the light to a light intensity detector. Strain in the substrate is transmitted by physical forces to the gallium arsenide chip which behaves optically according to the principles illustrated in FIG. 1. When the strain imposed on the chip causes compression of the chip, this is manifested as an increase in transmission of light through the chip; when the strain causes extension of the chip, a decrease in the intensity of light transmitted through the chip is observed.
FIG. 3 shows an example of experimental results obtained using the optical strain gauge configuration of FIG. 2. For the data shown, the median optical wavelength of the light passing through the sample was about 894 nm and its spectral width was about 4 nm. A significant fractional change in optical transmission is observed for very modest strain levels generated by mechanical loading.